Measurement and correction of optical aberrations in charged particle beam microscopy

Abstract

A charged particle beam microscope system is operated in a transmission imaging mode. During the operation, the charged particle beam microsystem directs a charged particle beam to the sample to produce images. A time series of beam tilts is applied in a pattern to the charged particle beam directed to the sample to produce a sequence of images. At least some of the images in the sequence of images are captured while the charged particle beam is transitioning between one beam tilt in the time series of beam tilts and a sequentially adjacent beam tilt in the time series of beam tilts. The pattern is configured to induce image changes between the images in the sequence of images that are indicative of optical aberrations in the charged particle beam microscope system.

The invention claimed is: 
     
       1. A method, comprising:
 with a charged particle beam microscope system operating in a transmission imaging mode, directing a charged particle beam to a sample including applying a time series of beam tilts in a pattern to the charged particle beam to produce a sequence of images, wherein at least some of the images in the sequence of images are captured while the charged particle beam is transitioning between one beam tilt in the time series of beam tilts and a sequentially adjacent beam tilt in the time series of beam tilts, and wherein the pattern is configured to induce image changes between the images in the sequence of images that are indicative of optical aberrations in the charged particle beam microscope system; and 
 capturing the sequence of images. 
 
     
     
       2. The method of  claim 1 , wherein the pattern is a periodic pattern. 
     
     
       3. The method of  claim 2 , wherein the periodic pattern is a Lissajous figure. 
     
     
       4. The method of  claim 1 , further comprising generating a set of one or more optical aberration values for the charged particle beam microscope system from the sequence of images. 
     
     
       5. The method of  claim 4 , wherein the image changes comprise induced image shifts, and wherein generating the set of one or more optical aberration values for the charged particle beam microscope system from the sequence of images comprises measuring the induced image shifts between successive images in the sequence of images. 
     
     
       6. The method of  claim 5 , wherein generating the set of one or more optical aberration values for the charged particle beam microscope system from the sequence of images further comprises fitting the induced image shifts to an optical aberration model describing a relationship between the induced image shifts, the beam tilts, and the set of one or more optical aberrations. 
     
     
       7. The method of  claim 6 , wherein the optical aberration model incorporates an unknown time delay between the pattern and the sequence of images. 
     
     
       8. The method of  claim 4 , wherein generating the set of one or more optical aberration values for the charged particle beam microscope system from the sequence of images comprises estimating a value for one or more of defocus, two-fold astigmatism, three-fold astigmatism, four-fold astigmatism, five-fold astigmatism, sixfold astigmatism, axial coma, fifth order axial coma, spherical aberration, sixth order spherical aberration, star aberration, sixth order star aberration, three-lobe aberration, and rosette aberration. 
     
     
       9. The method of  claim 4 , further comprising determining optical corrections for the charged particle beam microscope system based on the set of one or more optical aberration values. 
     
     
       10. The method of  claim 9 , further comprising applying the optical corrections for the charged particle beam microscope system to one or more optical components of the charged particle beam microscope system. 
     
     
       11. The method of  claim 4 , further comprising adjusting the pattern based on one or more of accuracy of the set of one or more optical aberration values, quality of the images in the sequence of images, and external disturbances while capturing the sequence of images. 
     
     
       12. A charged particle beam microscope system, comprising:
 a charged particle source that emits a charged particle beam; 
 an optical system configured to direct the charged particle beam toward a sample along an optical axis; 
 a pattern generator configured to generate a pattern; 
 one or more beam deflectors configured to apply a beam tilt to the charged particle beam upstream of the sample with the charged particle beam microscope system operating in a transmission imaging mode, wherein the one or more beam deflectors are controllable to apply a varying beam tilt to the charged particle beam according to the pattern to produce a sequence of images of the sample corresponding to the variations in the beam tilt such that at least some of the images in the sequence of images are captured while the beam is transitioning between different beam tilts; and 
 an imaging sensor positioned along the optical axis to capture the sequence of images of the sample, wherein the pattern is configured to induce image changes between the images in the sequence of images that are indicative of optical aberrations in the charged particle beam microscope system. 
 
     
     
       13. The charged particle beam microscope system of  claim 12 , wherein the charged particle beam microscope system further is configured to generate a set of one or more optical aberration values for the charged particle beam microscope system from the sequence of images. 
     
     
       14. The charged particle beam microscope system of  claim 13 , wherein the charged particle beam microscope system further is configured to:
 determine optical corrections for the charged particle beam microscope system based on the set of one or more optical aberration values; and 
 apply the optical corrections for the charged particle beam microscope system to one or more optical components of the charged particle beam microscope system. 
 
     
     
       15. The charged particle beam microscope system of  claim 13 , wherein the charged particle beam microscope system further is configured to adjust the pattern based on one or more of accuracy of the set of one or more optical aberration values, quality of the images in the sequence of images, and external disturbances while capturing the sequence of images. 
     
     
       16. One or more computer-readable storage media comprising stored instructions that, when executed by one or more processors coupled to a charged particle beam microscope system, cause the charged particle beam microscope system to:
 with the charged particle beam microscope system operating in a transmission imaging mode, direct a charged particle beam to a sample including applying a series of beam tilts to the charged particle beam according to a pattern to produce a sequence of images, wherein at least some of the images in the sequence of images are captured while the charged particle beam is transitioning between different beam tilts of the series of beam tilts, and wherein variations between the images in the sequence of images are correlated to the series of beam tilts in a manner that is indicative of optical aberrations in the charged particle beam microscope system; and 
 capture the sequence of images. 
 
     
     
       17. The one or more computer-readable storage media of  claim 16 , wherein the one or more computer-readable storage media further comprise stored instructions that, when executed by the one or more processors, cause the charged particle beam microscope system to generate a set of one or more optical aberration values for the charged particle beam microscope system based, at least in part, on the sequence of images. 
     
     
       18. The one or more computer-readable storage media of  claim 17 , wherein the one or more computer-readable storage media further comprise stored instructions that, when executed by the one or more processors, cause the charged particle beam microscope system to determine induced image shifts in the sequence of images and determine the set of one or more optical aberration values from the induced image shifts and the applied series of beam tilts corresponding to the induced image shifts. 
     
     
       19. The one or more computer-readable storage media of  claim 17 , wherein the one or more computer-readable storage media further comprise stored instructions that, when executed by the one or more processors, cause the charged particle beam microscope system to determine one or more optical corrections for one or more optical components of the charged particle beam microscope system based on the set of one or more optical aberration values. 
     
     
       20. The one or more computer-readable storage media of  claim 17 , wherein the one or more computer-readable storage media further comprise stored instructions t hat, when executed by the one or more processors, cause the charged particle beam microscope system to determine adjustments to the pattern based on one or more of an accuracy of the set of one or more optical aberration values, quality of the images in the sequence of images, and external disturbances while capturing the sequence of images.